This invention relates to a spark plug having a built-in resistor effective for suppressing noise signals.
There have been proposed spark plugs having various kinds of built-in resistors. For example, Japanese Patent Unexamined Publication No. 50-144830 discloses a spark plug comprising an insulator having a centerbore therethrough, a center electrode, and a resistor sealed together with conductive glass seals in the centerbore, said resistor being obtained by sintering a resistor powder mixture comprising tin oxide as a major resistor powder, an electrical insulating ceramic powder such as zirconia powder having a particle size of 177 .mu.m and a glass powder having a softening temperature of 300.degree. to 600.degree. C. Japanese Patent Unexamined Publication No. 57-105988 discloses a spark plug comprising an insulator having a centerbore therethrough, a center electrode and a resistor sealed together with conductive glass seals in the centerbore, said resistor being obtained by sintering a resistor powder mixture comprising an electrical insulating ceramic powder such as carbon black, zirconia, or the like, and two different kinds of glass powders. Further, Japanese Patent Unexamined Publication No. 61-104580 discloses a spark plug comprising an insulator having a centerbore therethrough, a center electrode and a resistor sealed together with conductive glass seals in the centerbore, said resistor being obtained by sintering a resistor powder mixture comprising a carbon powder, a glass powder having a larger particle size than that of carbon powder in the range of 5 .mu.m to 80 .mu.m, and a glass powder having a larger particle size than the former glass powder in the range of 50 .mu.m to 300 .mu.m.
But these spark plugs are insufficient in noise signal suppression effect.
The present inventors have studied causes of such insufficiency in noise signal suppression effect and found that boundary surfaces between the resistor and conductive glass seals sandwiching the resistor were curved to substantially reduce a resistance value due to substantial shortening of the resistor length, resulting in insufficient effect for suppressing noise signal.